Modbus provides Serial and Ethernet as communication methods. These communications methods use different data transmission modes: Remote Terminal Unit (RTU), American Standard Code for Information Interchange (ASCII), and Transmission Control Protocol (TCP).

  • Modbus Serial—At any time, only one Modbus master and one or more Modbus slaves are connected to the same serial bus.
  • Modbus Ethernet—At any time, one or more Modbus masters and only one Modbus slave are connected on the same Ethernet network.

    • The following table describes these transmission modes:

    Transmission ModeCommunications MethodDescription
    RTUSerialEach message data byte consists of two 4-bit hexadecimal characters. Devices and applications transmit each message in a continuous stream of characters.
    ASCIISerialEach message data byte consists of two ASCII characters.
    TCPEthernetThe Modbus data message is encapsulated in a TCP frame.